The present invention relates to methods and systems for producing L-ascorbic acid with reduced color. Specifically, the present invention relates to ascorbic acid with reduced color formed using palladium supported catalysts.
L-Ascorbic acid (vitamin C) is produced commercially by combined chemical and fermentation processes starting from glucose or sorbose. A common intermediate generated in the commercial process is 2-keto-L-gulonic acid (KLG), or its protected form, diacetone-2-keto-L-gulonic acid. The conversion of 2-keto-L-gulonic acid to L-ascorbic acid may be carried out by esterification with methanol, followed by cyclization using stoichiometric amounts of a base, in a methodology derived from the original Reichstein process (T. Reichstein, A. Grussner, Helv. Chim. Acta 17, pp. 311-328, 1934). Alternatively, diacetone-2-keto-L-gulonic acid may be cyclized directly, with a loss of acetone followed by consecutive lactonization and enolization, to form ascorbic acid. Direct cyclization of diacetone-2-keto-L-gulonic acid requires extensive purification for recovery of the acetone and other by-products generated.
Modifications to the Reichstein process have focused on removal or simplification of many of the chemical processing steps required for the production of 2-keto-L-gulonic acid. Improvements include controlled esterification of 2-keto-L-gulonic acid and subsequent removal of unesterified starting material (U.S. Pat. No. 5,128,487), as well as improved integration of esterification with subsequent cyclization (U.S. Pat. No. 5,391,770).
Efforts have also been directed to acid catalysis (e.g. U.S. Pat. No. 2,462,251; GB 1,222,322; GB 2,034,315; DE 19734086, DE 3843389; WO 99/07691; and WO 00/46216) thereby removing the steps of esterification with subsequent based-catalyzed cyclization and reprotonation of the L-ascorbic acid product. In addition, modifications to improve the process such as the use of organic solvents and surfactants have been described (see e.g. U.S. Pat. No. 5,744,618; WO 87/00839; and JP-B 73015931).
An alternative means of producing ascorbic acid from 2-keto-L-gulonic acid involves an aqueous intramolecular cyclization process without the use of copious amounts of acid catalysts (T. Reichstein, Helv. Chim. Acta 17, 1934, pp.311-328 and BP 428,815). Although aqueous cyclization does not require the extensive purification steps associated with acid catalysis, non-acid catalyzed intramolecular cyclization is associated with relatively low yields. For example, 2-keto-L-gulonic acid may be heated in water saturated with carbon dioxide with a 50% yield after fractional crystallization (U.S. Pat. No. 2,265,121). Also, 2-keto-L-gulonic acid or derivatives of 2-keto-L-gulonic acid may be heated to 130-140xc2x0 C. in water to generate ascorbic acid with yields approximating 50% (U.S. Pat. No. 2,491,065).
A common problem encountered with conversion of 2-keto-L-gulonic acid to ascorbic acid in water or acidic solutions is the production of colored solutions from degradation products that are formed during the reaction. These degradation products generally include high molecular weight compounds that accumulate as a function of conversion. Thus, with increasing conversion, solutions tend to become increasingly colored and eventually form insoluble by-products. Generally, methods to decolor ascorbic acid involve adsorption of the colored by-products using carbon or other solid supported agents. The use of large amounts of carbon or other solid decolorizing agents significantly hinders subsequent purification of the L-ascorbic acid product and ultimately, may become cost prohibitive.
Thus, there is a need for improved methods and agents to reduce the formation of color when ascorbic acid is produced by aqueous or acid catalysis. There is also a need for a heterogeneous catalyst that can be easily separated from the ascorbic acid product. Ideally, the agents used for reducing the formation of color employ materials that are relatively inexpensive, non-toxic, and easy to handle. Also ideally, the agent used contributes to catalysis of the reaction. Even more ideally, such color reducing catalysts are useable in systems that allow for separation of the catalyst from the reaction components. Also ideally, such color reducing should allow for recycling of unused starting compounds and/or catalyst when the synthesis is taken only to partial conversion, as for example, to avoid excessive degradation of ascorbic acid product.
The present invention describes the synthesis of ascorbic acid with reduced color using palladium (Pd) as a heterogeneous catalyst. The palladium catalyst may include an inert support such as, but not limited to, carbon or barium sulfate. Unlike other palladium catalysts, the catalyst of the present invention is active under aqueous conditions suitable for ascorbic acid synthesis.
Thus in one aspect, the present invention comprises a method for reducing the amount of color formed when ascorbic acid is synthesized from 2-keto-gulonic acid or 2-keto-gulonic acid derivatives comprising conducting the synthesis in the presence of palladium. In an embodiment, the palladium at least in part reduces the formation of colored byproducts in the reaction.
In another aspect, the present invention comprises ascorbic acid synthesized from 2-keto-gulonic acid or a 2-keto-gulonic acid derivative in the presence of palladium.
In yet another aspect, the present invention comprises a system for reducing the amount of color formed when ascorbic acid is synthesized from starting material comprising 2-keto-gulonic acid or a 2-keto-gulonic acid derivative comprising a reactor for conducting the synthesis in the presence of palladium.
The foregoing focuses on the more important features of the invention in order that the detailed description which follows may be better understood and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention which will be described hereinafter and which will form the subject matter of the claims appended hereto. It is to be understood that the invention is not limited in its application to the specific details as set forth in the following description and figures. The invention is capable of other embodiments and of being practiced or carried out in various ways.
From the foregoing summary, it is apparent that an object of the present invention is to provide methods and systems for synthesizing ascorbic acid from 2-keto-gulonic acid or a 2-keto-gulonic acid derivative to produce ascorbic acid that has relatively low levels of discoloration. More specifically, the present invention provides for the use of palladium as both a heterogeneous catalyst and as an agent to reduce the formation of color.
The present invention has several advantages over the prior art. For example, the palladium catalysts of the present invention provide for efficient synthesis of ascorbic acid in aqueous or acid conditions. The present invention may be used to prevent the formation of colored, high molecular weight byproducts and thus, improve the efficiency of the reaction and reduce purification costs. Also, as heterogeneous catalysts, the palladium catalysts of the present invention are easily removed from the ascorbic acid product. In addition, the palladium catalysis of the present invention may be used in established reactor formats such as stirred tank reactors, fluidized beds, or fixed bed reactors. These, together with other objects and advantages of the present invention along with various features of novelty which characterize the invention, are pointed out with particularity in the description, appended claims and accompanying drawings provided herein.